In recent years, large-capacity optical disks represented by DVD etc. are widely used. Upon recording information on the optical disk, an information-recording optical disk drive is used to focus a recording laser beam onto the information recording surface of the optical disk to form record marks thereon. Upon reproducing the information from the optical disk, information-reproducing optical disk drive is used to focus a reproducing laser beam onto the information recording surface of the optical disk to detect the reflected beam modulated by the record marks to reproduce the information. The optical disk allows the recorded information to be reproduced by another optical disk drive other than the optical disk drive that has recorded so long as the another optical disk drive is directed to the optical disk having the same specification.
Techniques for recording information onto the optical disk are categorized into two types including a mark position recording technique which allows the position of the record mark to have the information, and a mark edge recording technique which allows the front edge and rear edge of the record mark to have the information. Comparing both the techniques with each other, although the mark edge recording technique is suited to a higher recording density, the length of the record mark must be controlled with particularly higher accuracy.
In a rewritable optical disk, the record mark is generally formed by a temperature rise of the recording film due to irradiation of the laser beam. The length of the record mark changes depending on the disk configurations such as pigments or metals used in the optical disk and the track pitch, as well as the conditions such as the recording clock and the linear velocity. Accordingly, for controlling the record mark length with higher accuracy, it is necessary to suitably setting the recording condition including the pulse light intensity, pulse shape and pulse width (referred to as “recording strategy” hereinafter) of the laser beam irradiating the optical disk, depending on the configuration and linear velocity of the disk.
FIG. 17 shows waveform examples (a) to (d) of the laser output during forming record marks on the optical disk. Upon forming the record mark, the laser output is controlled based on one of the waveform patterns (a) to (d), to control the laser beam irradiating the recording surface of the optical disk. In the waveform (a), a laser beam having a constant output power is applied for the time length corresponding to the length of a record mark to form the record mark. In the waveform (b) and waveform (c), a pulse train including n or (n−1) pulses is used to form a record mark having a desired mark length. In the waveform (d), a non-pulse train having a higher laser output power corresponding to each of the front and rear edges of a record mark is used to form the record mark having a desired mark length and intensified front edge and rear edge.
In general, a record mark is formed to have a length which is an integral multiple of T, given T being a period of the clock (channel clock) used as a reference in the recording/reproducing operation. Each record mark is, for example, any one of the record marks having 2T to 8T lengths in a (1,7) RLL encoding scheme using a ⅔ conversion in the encoding. Here, 2T is constituted by a signal cycle including a space and a mark each having a length twice the period T of the channel clock, whereas 8T is constituted by a signal cycle including a space and a mark each having a length eight-time the period T of the channel clock.
In the optical disk drive, prior to recording on the optical disk, top width Ttop of the recording pulse, intermediate pulse width Tmp (or Tw), rear edge width Tlp of the recording pulse, rear edge cooling width Tcl set as a cooling control time, which are shown in FIG. 17, are determined as parameters for the recording strategy. Among other these parameters, the parameter strongly affecting pass or fail of forming the record mark differs depending on the kinds of the mediums, wherein some medium reveals a larger influence by the top width Ttop, some medium reveals a larger influence by the rear edge cooling width Tcl, some medium reveals a larger influence by the rear edge width Tlp, and some medium reveals a larger hybrid influence by the combination of these parameters. In the mark edge recording scheme, it is more often that the top width Ttop and rear edge cooling width Tcl are important parameters because the front and rear edges of the record mark must be suitably recorded.
In the optical disk drive, the recording strategy is generally determined based on a fixed rule, resulting from the constraints such as the circuit scale and the object medium. For example, if the recording is to be performed based on the strategy wherein the top width Ttop has a specified time length, it is determined that the record marks formed have the same value (corresponding to Ttop) for the top widths Ttop. Exemplifying the (1,7) RLL encoding scheme, the record marks 2T to 8T are recorded based on the recording strategy wherein the top pulse width Ttop and the rear edge cooling width Tcl are determined in common to the marks.
The techniques for suitably setting the recording strategy include a β technique which is employed in a DVD rewritable system such as DVD-R. FIG. 18 shows an example of the signal waveform used in the β technique. In the β technique, long mark and space (11T, for example) and short mark and space (3T, for example) are recorded while changing the recording condition, and then reproduced to calculate an asymmetry value (β value), thereby determining a recording condition including the recording strategy.
The β value is obtained by calculating the reference point Ref from the reproducing waveforms of the 11T and 3T record marks, and calculating the following formulaβ=0.5×(A−B)/(A+B)where A and B represent the peak level of 11T and the bottom level of 11T, respectively. The β value has a correlation with jitter σ which is an index for evaluating the recording signal quality. By setting the recording condition so that the β value falls within a specified range, a suitable record mark having a small jitter in the reproduced signal can be formed.
In general, if the jitter σ has a value of 15% or above, an error correction processing cannot correct an error in the reproduced signal. Thus, the jitter σ should be suppressed to 15% or below. For example, it is known that the correlation between the jitter σ and the bit error rate BER is such that the bit error rate BER=10−4 corresponds to the jitter σ≈12.8%. The factors for degrading the jitter σ include an error in setting the recording power, an error in setting the recording strategy, an error in setting the focus, an error in setting the tilt, and so on.
In a current optical disk drive, even if all the recording conditions are determined at optimum values, the obtained jitter σ is around 8% at the minimum. Since degradation of the jitter allowed for the errors in setting the recording power and the recording strategy is around 2%, it is preferable that the parameters of the recording power and recording strategy be determined during adjustment of the recording condition so that the jitter σ assumes 10% or below.
As a technique for optimizing the recording condition, Patent Publication JP-A-2000-30254 describes a technique wherein as the recording power, the theoretical mark length of the longest mark is determined as an index, whereas as for the recording strategy, a specific recording strategy providing a minimum value for the jitters of all the record marks or the jitter of the shortest record mark is selected.
As a high-density reproducing technique for the optical disk drive, a PRML (partial response maximum likelihood) technique is known wherein correction of the reproduced signal by partial response waveform equalization and detection of the maximum likelihood are combined together (proceedings of 1994 ITE Annual Convention (ITE' 94) of the Television Society, pp 287-288). In this technique, for maximizing the characteristics of the maximum likelihood detector in consideration of the reproduced channel, the reproduced signal is corrected based on the waveforms equalization and then subject to maximum likelihood detection. The use of the PRML technique has the advantage that a high-density reproducing performance can be assured without a sufficiently high resolution capability for the shortest mark.
FIGS. 19A and 19B show an example of the record data and an example of the reproduced signal waveform obtained from the record data, respectively. The solid line 22 in FIG. 19B represents a reproduced signal waveform in the case of higher recording density compared to the case shown by the dotted line 21. As the record mark length recorded on the disk is reduced along with the development of the higher density, the amplitude of the reproduced signal for the short record mark is reduced, as shown in FIG. 19B, whereby it happens that the amplitude may be lower than the signal slice level 23 used for detecting the mark position, although the record mark is actually formed. If the amplitude of the reproduced signal is below the slice level 23, there arises a problem in that the positions of the front edge and the rear edge of the record mark cannot be detected with accuracy.
For a record mark having a length near the optical diffraction limit, in particular, a sufficient amplitude cannot be obtained in the reproduced signal, and a larger influence by the inter-code interference occurs, whereby the detection accuracy of the jitter or asymmetry value is reduced. Thus, in the conventional techniques using the jitter or asymmetry value, including the technique described in the patent publication, the optimization of the recording condition directly by using the reproduced signal is difficult to obtain.
FIG. 20 show examples of the waveform of the reproduced eye-pattern obtained by recording/reproducing on a phase-change disk having an overcoat film thickness (substrate thickness) of 0.1 mm by using an optical disk drive mounting thereon an optical head having a laser waveform of 405 nm and an numerical aperture NA=0.85. FIG. 20A shows a reproduced eye-pattern in the case of recording at a recording density at which the shortest mark in the modulating codes provided a record mark length of 0.166 μm, FIG. 20B shows a reproduced eye-pattern in the case of recording at a recording density at which the shortest mark in the modulating codes provided a record mark length of 0.148 μm, and FIG. 20C shows a reproduced eye-pattern in the case of recording at a recording density at which the shortest mark in the modulating codes provided a record mark length of 0.125 μm. As shown in FIGS. 20A to 20C, as the record mark length of the shortest mark is reduced, i.e., as the recording density becomes higher, separation of the waveforms is difficult to achieve for the short mark at the amplitude center.
FIG. 21 shows the relationship between the shortest mark length and the jitter. It will be understood that the jitter in the reproduced signal increases, as the recording density is increased to reduce the shortest mark length in the modulating codes. In particular, it will be understood that the jitter exceeds 15% in the reproduced signal of the record mark having a shortest mark length, which is shorter than the mark length shown by the dotted line in the figure, wherein the ratio of the reproduced signal amplitude for the shortest mark in the modulating codes to the reproduced signal amplitude for the longest mark in the modulating codes is not larger than 10%. It is known that if the jitter exceeds 15%, the dispersion of each record mark becomes larger to exceed the window allowed for the each mark. This shows that separation of the reproduced signal for the short mark, in particular, is not sufficient to raise an inter-code interference and that the probability of mark generation which is detected in the case of a lower recording density cannot be detected any more using the jitter with high accuracy. It shows similarly that the detection accuracy of the asymmetry of the shortest mark is reduced in the β technique, and that an accurate judgement is not possible as to whether or not the record mark is suitably formed.
If the record mark is recorded with a higher accuracy to the extent where a sufficient amplitude of the reproduced signal is not obtained, then the PRML technique is effective. However, the PRML technique requires an additional PRML circuit to complicate the circuit configuration. In addition, since the bit error rate is calculated in the PRML technique by comparing the known data and the PRML detected recorded/reproduced data, the original data used for this comparison must be given data, and thus are difficult to handle in a general processing scheme unlike the case using the jitter or the asymmetry value.
It is an object of the present invention to solve the above problems and to provide a method for a recording condition setting method which is capable of determining the recording condition with simplicity and higher accuracy even in the case of recording at a high recording density to the extent where the reproduced signal of the shortest mark cannot be directly used, and to provide an information recording device using the same and a program.